Inheritance of Tomato Late Blight Resistance in Lycopersicon hirsutum
LA1033

Extensive efforts to control late blight of tomato and potato, caused by Phytophthora infestans (Mont.) de Bary, through incorporation of single resistance genes have been thwarted by the rapid development of virulent pathogen strains. In contrast, multigenic late blight resistance in potato has proven durable over decades of use. Lycopersicon hirsutum Kunth LA1033 is highly resistant to P. infestans, and other studies have suggested that this resistance is multigenic. We evaluated the inheritance of late blight resistance in LA1033 using traditional methods and quantitative trait analysis. A BC3F2 population was derived from L. esculentum Miller NC215E × L. hirsutum LA1033 and was screened for late blight resistance using detached leaf tests. BC3F3 progeny were evaluated in the field to verify detached leaf test results. Bulked segregant analysis was conducted on the most resistant and most susceptible BC3F2 plants to identify AFLP markers linked to resistance. A BC1F1 population of 284 individuals was also derived from L. esculentum NC215E × L. hirsutum LA1033. This population was evaluated for late blight resistance in the field and with detached leaf tests that were scored with a four-point rating system and the Horsfall-Barratt scale. A linkage map was constructed based on 155 BC1F1 individuals with 231 AFLP and two morphological markers. Quantitative trait analysis was conducted using the following four methods: simple linear regression, interval mapping, composite interval mapping, and multiple interval mapping. Segregation of late blight resistance in the BC3F2 individuals indicated that the BC3F1 plant carried two genes conferring late blight resistance; however, minor genes may have been lost in the development of the BC3F1 or may not have been detected. An AFLP marker was identified in the BC3F2 population which clearly segregated with resistance. In the BC1F1 population, segregation ratios indicated the presence of more than two late blight resistance loci. This was confirmed through quantitative trait analysis. Quantitative trait loci (QTL) affecting late blight response were detected on four to nine linkage groups depending upon the method of analysis used. The location of the most significant QTL corresponded with the marker identified through bulked segregant analysis.